The present invention relates generally to interferometers, and more specifically to techniques for stabilizing the alignment in a rapid scan Michelson interferometer used in a Fourier transform spectrometer.
A Fourier transform spectrometer typically includes a Michelson interferometer into which a broadband (typically infrared) beam to be analyzed and a monochromatic beam are directed. The interferometer has a fixed mirror and a movable mirror which is driven at a nominally constant velocity over a portion of its travel. Each of the input beams is split at a beamsplitter with one portion traveling a path that causes it to reflect from the fixed mirror and another portion traveling a path that causes it to reflect from the movable mirror. The portions of each beam recombine at the beamsplitter, and due to optical interference between the two portions, the intensity of the monochromatic beam and the intensity of each frequency component of the infrared beam is modulated at a frequency proportional to the components optical frequency and the mirror velocity.
The recombined beams are directed to appropriate detectors. The detector output for the infrared beam represents the superposition of these modulated components and provides an interferogram whose Fourier transform yields the desired spectrum. The monochromatic beam provides a nominally sinusoidal reference signal whose zero crossings occur each time the moving mirror travels an additional one quarter of the reference wavelength. The data acquisition electronics are triggered on these zero crossings to provide regularly sampled values for the interferogram. With the appropriate choice of mirror velocity, the output signal can be made to fall within a convenient range of modulation frequencies, as for example in the audio range. The mirror velocity can be stabilized by comparing the monochromatic output signal to a stable clock signal to produce an error signal, and applying a correction signal to the mirror drive so as to null the error signal.
A measure of the performance of an interferometer is the modulation efficiency, which is the strength of modulation (seen at the output) of the total input optical beam by a scanning interferometer. This is often measured as the ratio of the peak energy variation to the average output energy. The modulation efficiency is typically maximized by orienting the interferometer's optical elements, namely the beamsplitter, moving mirror, and fixed mirror such that the beamsplitter is located in the plane that bisects the planes defined by the fixed and the moving mirror when at zero retardation. This orientation also minimizes the sensitivity to small misalignments (transverse tilts) of the optical elements. Ideally, this orientation achieves uniformity of optical retardation within the entire cross section of the optical beam entering and exiting the interferometer.
It is known to correct for mirror misalignment relative to the beamsplitter by introducing a plurality of spatially separated monochromatic beams, typically two in addition to the main reference beam, each with a separate detector, providing actuators for tilting one of the optical elements (typically the fixed mirror), comparing the various monochromatic detector signals to each other to generate alignment error signals based on phase differences among the monochromatic detector signals, and applying correction signals to the actuators so as to null the alignment error signals.